Solving a mystery

Newly named soybean taproot decline disease continues to plague growers after a decade, but researchers are hot on the trail.

By Vicky Boyd Editor

Soybean taproot decline symptoms can be mistaken for SDS, or soybean death syndrome—Photo by Dr. Tom Allen

A soybean malady first noticed about 10 years ago causing plant mortality and dubbed the “mystery disease” was recently renamed soybean taproot decline. But researchers and consultants say many characteristics surrounding the fungal disease, such as the causal agent, disease origin and disease transmission, remain a mystery.

“I’ll tell you — I hope they find some way of controlling it,” says Brian Ward, a partner in Starkville, Miss.-based Southern Ag Consulting Inc. who first noticed the disease in Mississippi Delta soybean fields about 10 years ago. “I’m really glad (the researchers) are showing some interest in it. To me, I think it’s costing growers a lot of money.”

Tom Allen, a Mississippi State University Extension Service plant pathologist, has been studying the disease since he arrived at the university about nine years ago.

Once he and colleagues at the University of Arkansas and Louisiana State University have confirmed the organism’s identity, they can begin to study how it spreads and develop management strategies.

The researchers are 90 to 95 percent sure they know what organism is causing taproot decline, but they have yet to complete scientific verification tests, known as Koch’s postulates, to confirm their suspicions, Allen says.

Trey Price, an assistant professor of field crop pathology at LSU AgCenter’s Macon Ridge Research Station in Winnsboro, is part of the effort.

“I’m cooperating with Tom and Maria Tomaso-Peterson (Mississippi State) and Terry Spurlock at Arkansas, and we’re all trying to put our heads together to confirm pathogenicity,” Price says. “We suspect it’s a new disease that hasn’t been previously described.”

Allen did say the suspected causal agent is not related to the Fusarium complex responsible for sudden death syndrome (SDS) in soybeans, and it is not associated with soybean cyst nematodes, also tied to SDS.

“We’ve isolated organisms, and we are fairly certain it’s not in the Fusarium complex – we at least know that much at this point,” says Allen, who is based at MSU’s Delta Research and Extension Center in Stoneville.

Widespread throughout the Delta

Based on discussions with counterparts in other Southern soybean-producing states, Allen says soybean taproot decline appears limited to Arkansas, Mississippi and Louisiana. In those states, it is widespread.

Initially, leaves of plants infected with soybean taproot decline may have an almost pumpkin orange cast—Photo by Dr. Tom Allen

From his experience with the disease, Allen says he has seen no difference in varietal susceptibility or soil type.

“It’s in a lot of the varieties we grow,” he says, adding he has seen it on heavy clay soils and lighter loams. The disease also appears entrenched and is not just an anomaly.

“In the fields where we’ve observed the disease, it’s basically been there every year,” Allen says. “We’ve gone back to fields in continuous soybeans, and we’ve seen the disease in just about the same locations each year.”

Ward says he also has looked at various fungicidal seed treatments the past couple of years, but none seem to be effective.

Price says he saw more soybean taproot decline in fields in 2014, when growing conditions were wetter, than during the much drier 2015.

Based purely on visual observations, he says the disease appears more often and severe in continuous soybean fields with reduced tillage.

“What I’ve seen is in situations where you have soybean monoculture in a minimum or no-till program, incidence and severity of TRD are greater,” Price says.

In one center-pivot-irrigated field, for example, the grower planted half soybeans and half corn in 2014. The grower returned in 2015 to plant the entire field to soybean.

“In the area that had been planted to corn, there was significantly less TRD than in the area planted to soybean the previous year,” he says.

How much impact soybean taproot decline has on yield remains unanswered.

Price says he’s heard anecdotal reports from northeastern Louisiana growers about losses caused by the disease. But he stresses, “I need more information to get a handle on potential yield losses from it.”

As part of that effort, Price says he would like to find a few grower-cooperators this season so he could collect yield-loss data.

With some infected plants, the taproot will remain attached, but it may have a grayish to blackened mass growing around it, may have blackened lateral roots or is easily broken into two because of brittleness—Photo by Dr. Tom Allen

Taproot decline symptoms

Unlike SDS, which tends to begin producing symptoms at the beginning of flowering, taproot decline symptoms appear to become visible much earlier in the vegetative stages. However, the most characteristic symptoms associated with taproot decline are not observed until reproductive growth stages, which makes it similar to SDS in that respect.

Ward says many growers commonly mistake taproot decline for SDS, but symptoms of the two diseases are significantly different. Taproot decline begins as a bright orange to pumpkin-discoloration in the leaves that can be observed as early as vegetative stages in some fields.

But symptoms associated with taproot decline can be masked early by diseases, such as Septoria brown spot, that are a common early season disease in the lower canopy of continuous soybean fields.

“Immediately when I see that, I know what it is,” he says. As the disease progresses, it causes interveinal chlorosis. At times, green spots dot chlorotic leaves. Eventually the plant senesces and dies.

The telling symptom of taproot decline is if you carefully dig up a soybean plant, blackened tap and lateral root sections are evident. In some plants the majority of the taproot is missing.

In addition, if you split infected stems at the base of the plant, a white cottony growth is typically observed within the pith.

With some infected plants, the taproot will remain attached, but it may have a grayish to blackened mass growing around it, may have blackened lateral roots or is easily broken into two because of brittleness.